Nanowire Templated Semihollow Bicontinuous Graphene...

Nanowire Templated Semihollow Bicontinuous Graphene Scrolls:Designed Construction, Mechanism, and Enhanced Energy StoragePerformanceMengyu Yan,†,∥ Fengchao Wang,‡,∥ Chunhua Han,*,† Xinyu Ma,† Xu Xu,† Qinyou An,† Lin Xu,†,§

Chaojiang Niu,† Yunlong Zhao,† Xiaocong Tian,† Ping Hu,† Hengan Wu,*,‡ and Liqiang Mai*,†

†State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, WUT-Harvard Joint Nano Key Laboratory,Wuhan University of Technology, Wuhan 430070, People’s Republic of China‡CAS Key Laboratory of Mechanical Behavior and Design of Materials, Department of Modern Mechanics, University of Science andTechnology of China, Hefei, Anhui 230027, People’s Republic of China§Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, United States

*S Supporting Information

ABSTRACT: Graphene scrolls have been widely investigated for applications in electronics, sensors, energy storage, etc.However, graphene scrolls with tens of micrometers in length and with other materials in their cavities have not been obtained.Here nanowire templated semihollow bicontinuous graphene scroll architecture is designed and constructed through “orientedassembly” and “self-scroll” strategy. These obtained nanowire templated graphene scrolls can achieve over 30 μm in length withinterior cavities between the nanowire and scroll. It is demonstrated through experiments and molecular dynamic simulationsthat the semihollow bicontinuous structure construction processes depend on the systemic energy, the curvature of nanowires,and the reaction time. Lithium batteries based on V3O7 nanowire templated graphene scrolls (VGSs) exhibit an optimalperformance with specific capacity of 321 mAh/g at 100 mA/g and 87.3% capacity retention after 400 cycles at 2000 mA/g. TheVGS also shows a high conductivity of 1056 S/m and high capacity of 162 mAh/g at a large density of 3000 mA/g with only 5 wt% graphene added which are 27 and 4.5 times as high as those of V3O7 nanowires, respectively. A supercapacitor made of MnO2nanowire templated graphene scrolls (MGSs) also shows a high capacity of 317 F/g at 1A/g, which is over 1.5 times than that ofMnO2 nanowires without graphene scrolls. These excellent energy storage capacities and cycling performance are attributed tothe unique structure of the nanowire templated graphene scroll, which provides continuous electron and ion transfer channelsand space for free volume expansion of nanowires during cycling. This strategy and understanding can be used to synthesizeother nanowire templated graphene scroll architectures, which can be extended to other fabrication processes and fields.

■ INTRODUCTION

Graphene, the first two-dimensional (2D) atomic crystalstructure available to us, shows great mechanical stiffness,1,2

strength and elasticity,3 very high electrical4,5 and thermalconductivity,6 and large surface area.7 The outstandingproperties of graphene have motivated intensive efforts toconstruct graphene-based materials, which have been widelyused in energy storage8−10 and energy conversion11,12 devices,sensors,13 catalysis,14,15 and bioapplication.16,17 Most of theactive materials are deposited on the surface of the grapheneframework (2D, 3D) or encapsulated as a core in the grapheneshell (0D). Nevertheless, graphene-based materials with well-

defined 1D and tens of micrometers in length have not beenrealized.1D nanostructures are important in materials science and

technology, since they have some unique properties associatedwith the bridge between micro- and nanoscale features18 andthe confinement effect in the axial direction.19,20 Graphenescrolls (GSs) are 1D carbon materials normally formed througha graphene rolling mechanism which exhibits the theoreticalpossibility of encapsulating nanodroplets21 or other nanoma-

Received: August 31, 2013Published: November 12, 2013

Article

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© 2013 American Chemical Society 18176 dx.doi.org/10.1021/ja409027s | J. Am. Chem. Soc. 2013, 135, 18176−18182

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terials into the interior cavities22−24 due to the topology of theGSs. GSs provide open structures at both ends and interlayergalleries that can be easily intercalated and adjusted, whichshow great potential applications in energy storage.25−27 Themost important is that GSs, formed by the roll-like wrapping ofgraphene sheets, enable wrapping nanowires/nanotubes in thescrolls in theory (Figure S1, Supporting Information). It wascalculated that nanowires can activate and guide the self-assembly of planar graphene sheets on the nanowire/nanotubesurface, which showed lower energy demands than theformation of pure GSs.28−30 However, such a self-assemblyprocess has not yet been achieved in the experiments. Inaddition, the length of GSs, limited by the scale of graphene(less than 5 μm in traditional cases),31 is another challenge forthe application of GSs in energy storage, sensors, andnanoelectronics.32−39 Longer GSs may provide novel oppor-tunities for enhancing transport electrical and other physicalproperties through nanoscrolls over large length scales.In this paper, V3O7 and MnO2 are chosen as prototypes to

demonstrate the “oriented assembly” and “self-scroll” constructmechanisms of nanowire templated graphene scrolls. There aretwo key dynamics processes in the self-assembly. The first is therapid growth of nanowires and oriented assembly of thegraphene sheets on the surface of the nanowire and then theself-scroll of graphene ribbons (Figure 1). Such a facile method

can break the length limitation of GSs determined by the size ofgraphene (GSs over 30 μm in length were achieved).Nanowires being rolled into the hollow cavities of GSs showa new synthesis strategy to introduce other materials into thecavities and greatly widen the application range of GSs or GS-based composite materials. Further experiments and moleculardynamics (MD) simulations results confirm the structure andconstruction mechanism of the V3O7 nanowire templatedsemihollow bicontinuous graphene scroll (VGS) architecture.The VGSs offer a unique combination of ultralong 1Dgraphene scrolls with semihollow structure and space confiningeffect for the nanowires without self-aggregation during cycling,thus allowing for a greater lithium ion storage capacity andstability. As a proof of concept, a cathode made of VGSs showsa high specific capacity of 321 mAh/g at 100 mA/g and leads toa great enhancement of cycling stability and rate capability withonly 5 wt % graphene added. Furthermore, the orientedassembly and self-scroll strategy is also used in designing theconstruction of MnO2 nanowire templated graphene scroll(MGSs), which show morphology characteristics and enhancedenergy storage performance similar to those of VGS. On the

basis of the same synthetic strategy, other matters can play thesame roles as the template of the graphene scroll.

■ EXPERIMENTAL SECTIONMaterials and Methods. The rGO was synthesized through a

modified Hummer method.40,41 The hydrothermal method was usedto synthesize the VGS with the as-prepared rGO suspension (∼1 mg/mL). A 1.3 mmol portion of vanadium sol and aniline were mixed in a100 mL beaker and stirred at room temperature for 30 min in a molarratio of 1:0.03. Next, 13 mL of an rGO suspension (∼5 wt %) wasadded to the mixture, which was then stirred at 25 °C for 1 h. Thesample was then placed into a 100 mL autoclave and heated at 180 °Cfor 12−96 h. After washing and drying, a blackish green powder wasobtained. The pure V3O7 nanowires (PV) were prepared through thesame method above without addition of rGO. The V3O7 nanowire/graphene structure without nanowire templated graphene scrolls (VG)was prepared by mixing 13 mL of an rGO suspension with 1.3 mmolof PV and then drying at 80 °C for 24 h. The synthesis details of rGO,PV, and MGS and fabrication details of electrochemical devices arefurther described in the Supporting Information.

Characterization. An X-ray diffraction (XRD) measurement wasperformed to investigate the crystallographic information using a D8Advance X-ray diffractometer with a non-monochromated Cu Kα X-ray source. Field-emission scanning electron microscopic (SEM)images and energy dispersive spectroscopic (EDS) line scans werecollected with a JSM-7001F instrument at an acceleration voltage of 10kV. Transmission electron microscopic (TEM) and high-resolutionTEM images were recorded with a JEM-2100F STEM/EDSmicroscope.

MD Computational Details. All of the MD simulations in ourpresent work were implemented in the large-scale atomic/molecularmassively parallel simulator (LAMMPS).42 Simulations of thewrapping process of a nanowire by a monolayer graphene ribbonwere carried out for 500 ps with a time step of 1.0 fs. The grapheneribbon was constructed with the length and width of 42.6 nm. Theinteractions among the carbon atoms in the graphene were describedby the adaptive intermolecular reactive empirical bond order(AIREBO) potential.43 The interactions between graphene and 1-nanometer-diameter nanowires were modeled by a Lernard−Jones(LJ) potential with a cutoff of 12 Å. LJ parameters were simply set tobe ε = 5.0 meV and σ = 3.0 Å, respectively. After the initialconfiguration was constructed, the model was equilibrated at T = 450K with a Langevin thermostat for temperature control. During the runtime, simulation snapshots were saved every 1.0 ps for postsimulationanalysis.

■ RESULTS AND DISCUSSIONWe choose V3O7 as the first prototype to demonstrate theconstruction, mechanism, and energy storage performance ofnanowire templated semihollow bicontinuous graphene scrolls.The crystal structures have been characterized by XRDmeasurement. The obtained VGS and PV are both indexedto hydro V3O7 (028-1433) (Figure S2, Supporting Informa-tion). SEM and TEM observations were further employed tocharacterize the morphology and detailed structure of theproducts. It is important to note that the sizes of graphenesheets are only 1−2 μm. Notably, the low-magnification SEMimage (Figure 2a) shows that the length of synthesized VGS iscontinuous over 30 μm, which is much longer than the scale ofgraphene. Focusing on the end of the V3O7 nanowire (Figure2b) and GSs (Figure 2c), the V3O7 nanowires have been totallyintroduced into the cavity of the GSs. The length of V3O7nanowires is 1−2 μm shorter than that of GSs, which equals thesize of graphene. This phenomenon proves that the graphenesheets are assembled with the support of V3O7 nanowires. ATEM image of the synthesized nanowire (Figure 2d)demonstrates that the diameter of V3O7 nanowires is less

Figure 1. Construction processes of nanowire templated graphenescrolls. The green dots represent the precursor of the nanowiretemplate and then formation into the nanowire. The gray sheets andscroll represent the reduced graphene oxide (rGO).

Journal of the American Chemical Society Article

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than that of VGS with hollow channels. The lattice spacings of0.483 and 0.468 nm match well with the separations between(130) and (200) planes, respectively. The V3O7 nanowiresshow a preferred {130} growth orientation. Transferring to theedge of VGS (Figure 2e), the specific lattice fringe of 7−10layers of graphene can be observed with a lattice spacing of0.734 nm. Furthermore, we can find an overlap between rGOsheets. According to the circumference formula, this indicatesthat at least two pieces of graphene sheets had rolled around

the V3O7 nanowire in each segment of the VGS (details aredescribed in the Supporting Information). An EDS line scan(Figure 2f) further confirms that the V3O7 nanowires are in theGS cavity with obvious semihollow tubes.The construction and formation mechanisms of the VGS

have been investigated through a joint experimental−MDsimulation. The growth mechanisms of oriented assembly andself-scroll are proposed to demonstrate the complicatedarchitecture processes. The oriented assembly mechanism

Figure 2. Structure of VGS: (a) SEM image of synthesized VGS at low magnification; (b, c) SEM images of the end of the V3O7 nanowire and theend of GSs in VGS, respectively; (d) TEM images of VGS (the inset gives an HRTEM image of a V3O7 nanowire in GSs); (e) TEM image of theedge of VGS, showing a multilayer graphene coating; (f) SEM image and corresponding EDS line scan of VGS, showing the scanning route (greenline), the element distribution of vanadium (red line), and carbon (yellow line), respectively.

Figure 3. Formation mechanisms of VGS: (a−c) SEM images of VGS with hydrothermal reaction times of (a) 12 h, (b) 24 h, and (c) 48 h; (d)change in per-carbon energy during the wrapping of a graphene sheet onto a nanowire to form a GS (the insets are a series of simulationconfigurations of the formed GSs); (e) full view, partial enlarged view, and cross-section view of the MD snapshot of the nanowire templatedgraphene scroll semihollow structure.



describes the spontaneous self-organization of adjacentmaterials, so that they can react with each other, followed bythe joining of these materials at a planar interface. In thereaction, V3O7 nanowires form for the strong orientationgrowth of {130} in a short time (less than 12 h in this case).Then, the oriented assembly can guide the graphene sheets toassemble on the V3O7 nanowire surface and self-assemble to agraphene nanoribbon (Figure 3a). Later, the graphenenanoribbon tends to scroll surrounding the V3O7 nanowires,leading to a decrease in the graphene ribbons’ width. Thediameter of the graphene ribbon/scroll is calculated throughstatistics and decreases from over 200 nm (Figure 3b, c) to lessthan 150 nm (Figure S3, Supporting Information) with anincrease of reaction time. The self-scroll process can bededuced through the decrease of graphene scroll diameter withincreasing reaction time. Further information about VGS isobtained by taking the SEM image along one VGS (Figure S4,Supporting Information). We find that two V3O7 nanowires arerolled in a GS with semihollow channels in it. The diameter ofVGS is reduced by half, accompanied by the disappearance ofone V3O7 nanowire in the cavity of the VGS. Meanwhile, therelative position of the two V3O7 nanowires is ever-changing inthe GSs. It is demonstrated that the diameter of the VGS andthe hollows in GSs are related to the space structure and thediameter of V3O7 nanowires.Generally, rGO sheets dispersed in solution exhibit a trend

toward agglomeration. However, the above experimentalobservation indicated that a graphene ribbon formed alongthe nanowire before it scrolled and wrapped the nanowire(Figure 3a). This argument is supported by further MDsimulations using the reactive force field (ReaxFF) poten-tial.44,45 The results indicate the formation of covalent bondsbetween the rGO sheet and vanadium oxides, which anchoredthe graphene sheets to the V3O7 nanowire and facilitated theformation of the graphene ribbon (Figure S5, SupportingInformation). Details of the simulations are described in theSupporting Information. MD simulations have also beenperformed to investigate the wrapping process of a nanowireby a monolayer graphene ribbon (Supporting Information,movie 1). Formation of the VGS involves competition amongthe bending energy of the graphene ribbon, the graphene−nanowire interaction energy, and the graphene−grapheneinterlayer interaction energy.23,46,47 The graphene sheet startsto wrap the nanowire due to the graphene−nanowireinteraction energy, which increases the bending energy of thegraphene ribbon. After the first layer of the GS was formed, thewrapping process proceeded spontaneously, since the lowerenergy states indicate that the GSs are more stable (Figure 3d).During the simulations, the system was equilibrated at T = 450K, which is consistent with experiment. As far as we know, theinterior cavity in the nanowire templated graphene scrollnanoarchitecture has never been reported in previousstudies.22,23,28,45 According to the experimental observation,the V3O7 nanowires are wrapped in GSs with some curves, asshown in Figure S4 (Supporting Information). To clarify howthe observed nanowire templated graphene scroll nanostructurewith a semihollow cavity is formed, various simulations arecarried out with straight nanowires as well as curved nanowireswhich are preset to have a certain bending structure. We findthat when the graphene ribbon scrolls and wraps a nonstraightnanowire, an interior cavity is formed (Figure 3e). However,the hollow structure cannot be found in the simulations of thenonstraight nanowire wrapped by a single graphene sheet or of

the straight nanowires wrapped by a graphene ribbon (FigureS6, Supporting Information).On the basis of the experimental data and simulation results,

we propose the mechanism of the oriented assembly and self-scroll processes as follows. First, the competing growth reactionof V3O7 between the homogeneous nucleation in solution andheterogeneous nucleation on graphene sheets plays animportant role. In this step, high vanadium sol concentrationand rapid heating are needed to ensure that the homogeneousnucleation dominates the reaction process, which results in therapid growth of V3O7 nanowires. Second, the bonding of thegraphene and nanowire leads to a more steady energy statethan the agglomeration of the graphene sheets does.Consequently, the graphene sheets are anchored on the surfaceof the nanowires and form a ribbon. When the graphene ribbonwraps a curved nanowire, the film tension inside the ribbon willpromote the formation of GSs with hollow cavities rather thantight wraps of the GSs onto the nanowire. Third, the edge ofthe graphene ribbon starts to wrap the nanowire due to anattractive interaction between the graphene and nanowire.Fourth, the graphene ribbon bends along the surface of thenanowire and forms the GSs. Because of the curved nanowirestructural template, the GSs cannot become tightly cocoonedonto the nanowire. To reduce the bending energy along thenanowire, the GSs enlarge the diameter of the scroll core andform a semihollow structure. As shown in Figure 3e, D is thediameter of the VGS, which is the sum of the diameter of thenanowire d, the width of the semihollow channel w, and thethickness of the graphene multilayers h. MD results indicatethat the width of this hollow channel w is determined by thebending deflection of nanowires s (Figure S7, SupportingInformation): i.e., w ≈ s. If s ≥ 2.5 d, the nanowire cannot befully wrapped by the graphene ribbon (Figure S8, SupportingInformation). On the basis of experiments and calculations, h isa small value in comparison with D. Thus, we can propose arelationship to estimate the diameter of the VGS, D = d + s(Supporting Information, part 2). Consequently, a differentwidth of the semihollow can be achieved by controlling the sizeof graphene, the reaction time, and the curvature of nanowire.The approaches above enabled us to fabricate lithium

batteries with bicontinuous electron and lithium ion transferchannels in the electrode material, as shown in Figure 4a.During lithiation, electrolyte can enter the inner hollow spacedue to the capillarity effect48,49 and the lithium ion can reactwith the inner V3O7 nanowire directly. Meanwhile, the VGSswill bend and wrap with each other to relax the strain energy.During delithiation, the V3O7 nanowires shrink back. The VGSstructure provides three attractive features as an electrodematerial in comparison to pure V3O7 nanowires: (1) thesemihollow channels in the GSs provide space for free volumeexpansion of V3O7 nanowires and a continuous lithium iontransfer channel; (2) the GSs form a continuous electrontransfer channel during charging and avoid the agglomerationof V3O7 nanowires; (3) the semihollow channel of GSs canlimit the dissolution of V3O7. At a current density of 2000 mAg−1, the initial discharge capacities of VGS and VG (simplymixed with graphene; Figure S9b, Supporting Information) are158 and 140 mAh/g, respectively. The discharge capacitieschange to 138 and 100 mAh/g after 400 cycles, correspondingto capacity retentions of 87.3% and 71.4%, respectively (Figure4b). The PV, without mixing with graphene (Figure S9a,Supporting Information), shows a poor capacity that is lowerthan 50 mAh/g. The TEM images of VGS show that the



structure of the VGS can be kept intact even after 400 times ofcycling at 2 A/g (Figure S10, Supporting Information). Thedischarge capacities are measured in the voltage window from1.5 to 4 V at current densities of 100, 500, 1000, 2000, and3000 mA/g. VGS, VG, and PV all show high capacities of 321,290, and 267.9 mAh/g at 100 mA/g (Figure S11, SupportingInformation). At a very high current density of 3000 mA/gVGS still maintains a high capacity of 162 mAh/g, which is 1.7and 4.5 times higher than those of VG and PV, respectively(Figure 4c). EIS is employed to understand why the VGSshows a better electrochemical performance (Figure 4d). Thecharge transfer resistances (Rct) of VGS, VG, and PV are 32, 63,and 81 Ω, respectively. Further information is given by testingthe I−V characteristics of a single nanowire (Figure 4e). Withan effective length and cross section of the VGS atapproximately 4.98 μm and 89.52π nm2, respectively, wecalculate the conductivity value to be as high as 1056 S/m,which is 27 times higher than that of PV. This value can rangefrom 800 to 1500 S/m depending on the different lengths anddiameters of the samples. The above results demonstrated that(1) the electrochemical performance of V3O7 nanowires isenhanced with only 5 wt % graphene sheets added, (2) thegraphene sheets scrolled on the surface of the nanowire aremuch more efficient than simply mixed sheets with nanowires

for enhancing the conductivity, (3) the VGS shows bettercycling stability than VG due to the hollow channel and stablestructure of the nanowire templated graphene scroll architec-ture.To further identify the oriented assembly and self-scroll

strategy, the MGS was constructed through the hydrothermalmethod. The crystal structures of MGS have been characterizedby XRD measurements, which show that the MnO2 nanowireand MGS are indexed to α-MnO2 (072−1982) (Figure S12,Supporting Information). The SEM observations indicate thatultralong MGSs have been obtained (Figure 5a), which is

consistent with the VGS results. The TEM images of the MGS(Figure 5b) demonstrate that the MnO2 nanowires have beenscrolled into the graphene scroll. The lattice spacings of 0.272and 0.313 nm match well with the separations between (101)and (310) planes, respectively. At the edge of the MGS (Figure5c) the graphene layer can be observed clearly. Then, theelectrochemical performance of pure α-MnO2 nanowires andMGSs were tested by fabricating a supercapacitor (Figure 5d).On the basis of the discharging curve line, the specificcapacitance of the MGS is calculated to be 317 F/g at 1 A/g,which is over 1.5 times greater than that of the MnO2 nanowire(194 F/g). It is demonstrated that the nanowire templatedsemihollow bicontinuous graphene scrolls can improve theperformance of material in other systems as well.

■ CONCLUSIONSemihollow bicontinuous VGS and MGS over 30 μm in lengthhave been obtained. The construction processes and therelationship among the width of the semihollow, the curvatureof the nanowire, the size of graphene, and the reaction time aredefined and explained by combining experiments and MDsimulations. The unique structure of VGS provides space for

Figure 4. Electrochemical and electrical characterization of VGS. (a)Schematic of the VGS nanoarchitecture with continuous electron andLi ion transfer channels. The nanowire templated graphene scrollnanostructure provides internal void space for swelling duringlithiation and an effective internal electrolyte channel to enhance theion diffusion coefficient. In a pure nanowire structure, the strain couldnot be completely and promptly released. The nanowires tend toaggregate during cycles, leading to poor cycling performance. (b)Galvanostatic discharge profiles of VGS (red line), VG (black line),and PV (blue line) at 2000 mA/g tested between 4 and 1.5 V. (c)Galvanostatic discharge profiles of VGS, VG, and PV cycled at variouscurrents from 100 to 3000 mAh/g tested between 4 and 1.5 V. (d)Impedance measurements for VGS, VG, and PV at the initial state. (e)Single-nanowire transport properties of VGS and PV (the inset is theSEM image of a VGS single-nanowire device).

Figure 5. Structural and electrochemical characterization of MGS: (a)SEM image of synthesized MGS at low magnification; (b) TEMimages of MGS (the inset is an HRTEM image of MnO2 nanowire inGSs); (c) TEM image of the edge of MGS, showing multilayergraphene coating; (d) Galvanostatic charge/discharge curves measuredat a constant current density of 1 A/g for MnO2 nanowire and MGS in2 M KOH.



the volume expansion and shows the space confining effect forinhibiting the agglomeration of V3O7 nanowire, thus leading toremarkable electrochemical properties. Lithium ion batterycathodes made of VGS exhibit a high reversible capacity of 321mAh/g at 100 mA/g and 87.3% capacity retention after 400cycles at 2000 mA/g. Even at a high current density of 3000mA/g, a reversible capacity of 162 mAh/g can be achieved withonly 5 wt % graphene added, which is 4.5 times as high as thatof pure V3O7 nanowires. The supercapacitor made of MGS alsoshows a high capacity of 317 F/g at 1 A/g, which is over 1.5times greater than that of the MnO2 nanowire. These resultsdemonstrates that constructing VGS by combining orientedassembly and self-scroll is a significant and facile route toimprove electrochemical properties with low quantities ofgraphene. The nanowire templated graphene scroll nano-architecture described in this paper will be a unique structurethat has potential applications in energy storage and otherfields.

■ ASSOCIATED CONTENT*S Supporting InformationText, figures, and an AVI file giving procedures and additionaldata. This material is available free of charge via the Internet athttp://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*[email protected]; [email protected]; [email protected].

Author Contributions∥These authors contributed equally.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the National Basic ResearchProgram of China (2013CB934103, 2012CB933003), theNational Natural Science Foundation of China (51302203,51272197 and 11172289), the International Science &Te c h n o l o g y Co o p e r a t i o n P r o g r am o f Ch i n a(2013DFA50840), the Program for New Century ExcellentTalents in University (NCET-10-0661), and the FundamentalResearch Funds for the Central Universities (2012-II-001,2012-YB-02). Thanks are due to Professor C. M. Lieber ofHarvard University and Professor Q. J. Zhang of WuhanUniversity of Technology for strong support and stimulatingdiscussions. Special thanks are given to J. Liu of PacificNorthwest National Laboratory for his careful supervision,strong support, and stimulating discussions.

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